Move helper for getting a terminating musttail call to BasicBlock

No functional change.  To be used in future commits that need to look
for such instructions.

Reviewed By: rafael

Differential Revision: http://reviews.llvm.org/D4504

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215413 91177308-0d34-0410-b5e6-96231b3b80d8